Various research and proposals have been made on an exhaust gas purification device to purify exhaust gas of an internal combustion engine such as a diesel engine and some gasoline engines. In these, an exhaust gas purification device is used in which a DPF (diesel particulate filter) and a NOx purification catalyst that purifies NOx (nitrogen oxides) are arranged. A three way catalyst, a NOx adsorption catalyst, an SCR catalyst (selective contact catalyst) in which urea is added, a NOx direct reduction type catalyst, etc. are used as the NOx purification catalyst.
Then, an oxidization catalyst is arranged in the upstream side of the DPF or NOx purification catalyst in the exhaust gas purification device of a diesel engine. Then, the temperature of the oxidization catalyst and the temperature of the exhaust gas in the downstream side of the oxidization catalyst is increased in the case that the temperature of the exhaust gas is low by supplying a reductant such as HC into the exhaust gas with a post injection or an exhaust pipe injection and oxidizing this reductant with the oxidization catalyst. With this increase of temperature, the temperature of this oxidization catalyst is kept at an activation temperature or more, and at the same time, PM combustion in the DPF in the downstream side is promoted. Further, the temperature of the NOx purification catalyst in the downstream side is kept at the activation temperature or more.
Further, a reaction from NO to NO2 is promoted with this oxidization catalyst in an exhaust gas purification device in which the oxidization catalyst is arranged in the upstream side of the SCR catalyst. With this, a reaction with NH3 (ammonia) on the SCR catalyst is promoted.
This oxidization catalyst exhibits an action effect such as oxidizing CO, HC, and NO in addition to increasing the temperature of the exhaust gas, and also has a role to secure the promotion of combustion of PM in the DPF in the downstream side by NO2, and the performance of the NOx purification catalyst. The configuration, the composition, and the carrying amount of this oxidization catalyst are selected that are appropriate for each usage.
Further, a NOx adsorption catalyst is carried on the DPF without providing the oxidization catalyst in the upstream side as described in Japanese Patent Application Kokai Publication No. 2004-44515. In this system, a reductant such as HC is supplied into the exhaust gas with a post injection or an exhaust pipe injection in the case that the temperature of the exhaust gas is low. The temperature of the DPF with a catalyst is increased through oxidizing this reductant with the NOx adsorption catalyst, and the PM combustion in the DPF is promoted.
Then, the supply of the reductant in these devices is controlled with an activation temperature of the catalyst such as an oxidation catalyst carried in the exhaust gas purification device as a standard. That is, when the temperature of the exhaust gas flowing into this catalyst or the temperature of the catalyst is at the activation temperature or less, stops the supply of the reductant is stopped and the supply of the reductant is begun when the temperature becomes in a state of exceeding the activation temperature.
However, in the conventional art, the supply period and the supply amount of this reductant are in control as a function of only the temperature of the catalyst such as an oxidation catalyst or the temperature of the exhaust gas, and it was considered to have no relationship with the oxygen concentration, the CO concentration, and the HC concentration that give large influence to the oxidization of the reductant. Because of that, an appropriate amount of the reductant cannot be supplied at an appropriate period. As a result, there has been a problem that the performance of increasing the temperature of the catalyst cannot be sufficiently exhibited and a problem that mileage deterioration and deterioration of the exhaust gas component occur due to supplying an extra amount of the reductant.
On the other hand, an exhaust purification device in which the oxygen concentration in the exhaust gas is considered when supplying the reductant has been proposed. For example, as described in Japanese Patent Application Kokai Publication No. 2002-195074, an exhaust purification device of an engine as following is proposed in which NOx is purified by supplying the reductant consisting of HC, etc. into the exhaust gas with a post injection and reacting this reductant with NOx by using the NOx purification catalyst device. In this device, whether the NOx purification catalyst is in an active state or not is determined by the catalyst temperature, etc. With this determination result, the injection amount and the injection period of the post injection are changed, and the NOx discharging amount into the exhaust gas is made less in an inactive state. Further, the injection amount and the injection period of the post injection are set so that the reductant amount supplied to the NOx purification catalyst can be sufficiently secured in an active state. With this, the NOx release into the atmosphere is effectively suppressed, and the exhaust gas can be purified.
Further, an exhaust purification device of an internal combustion engine as described in Japanese Patent Application Kokai Publication No. 10-252544 has been proposed for example. In this device, the HC supply amount actually supplied into the exhaust gas is estimated based on the oxygen concentration in the exhaust gas in the case of supplying HC into the exhaust gas with a post injection. The post injection is controlled so that the actual HC supply amount is agreed with the objective value of the HC supply amount to be supplied.
However, the relationship of the oxygen concentration with the activation temperature characteristics (light off characteristics) of the catalyst is not referred to in these exhaust purification devices, and nothing more than the oxygen concentration is used only in order to estimate the HC supply amount actually supplied into the exhaust gas.    Patent Document 1: Japanese Patent Application Kokai Publication No. 2004-44515    Patent Document 2: Japanese Patent Application Kokai Publication No. 2002-195074    Patent Document 3: Japanese Patent Application Kokai Publication No. 10-252544